Hot forging is a process by which the shape and physical properties of metal can be changed. The process involves placing a piece of metal (normally heated) between the halves of a die and forcing the die to close by impact or pressure. The operation causes a controlled plastic deformation of the metal into the cavities of the die. This flow of material results not only in a change in shape of the metal but also increases the density and uniformity of the metal, improves its grain structure, and causes a shape-conforming grain flow. The resulting workpiece has properties which are superior to those generated by other methods, making forging essential where high performance workpieces are required.
One of the critical components of a forging system is the lubricant which separates the die from the workpiece. As with all lubricating situations, it is essential that this lubricant be effective to minimize wear of the extremely expensive forging dies and minimize expenditure of energy over a wide range of conditions.
The lubricant must also assure a high quality surface on the forging and not leave objectionable residues or corrosion on the dies.
As modern demand for safer and more dependable machine structures increases, the forging art is being applied to more difficult materials at higher temperatures and pressures to form more complex shapes. Although oil-based lubricating compositions, which are effective under these extreme conditions, have been developed, their properties are found to conflict seriously with national commitments to personal safety and protection of the environment. The oil-based lubricants are normally flammable under and can ignite well below common operating temperatures. Normal operation results in billowing carbonaceous smoke which is unpleasant and sometimes toxic. Furthermore, cleaning of the workpieces and dies requires solvent washes that produce large quantities of rinse, which, because of the economics of recycling and the desire to protect the environment, can present serious disposal problems.
The ecological problems associated with oil-based hot forging lubricants have led to the development of water-based compositions. One obvious advantage of a water-based lubricant composition is that die cooling can be accomplished by water evaporation on the hot dies, often making separate water cooling of the dies unnecessary. Early attempts directed to water-based compositions involving graphite, clay minerals, iron oxide and molybdenum disulfide E.P. and anti-wear additives were often ineffective because the water did not adequately wet the hot die surfaces.
One of the early disclosures of water-based lubricants is U.S. Pat. No. 2,735,814 wherein a die forging lubricant contained fish oil, graphite and water. The patentee in U.S. Pat. No. 2,921,874 employed fatty acids, combined with an organic acid reactant, such as phthalic acid, a solvent and water as lubricants for cold forming operations.
The patentee in U.S. Pat. No. 3,313,729 discloses a mixture of pyrophosphate or sodium tetraborate and a fatty acid soap of 8 to 22 carbon atoms, preferably 12 to 18 carbon atoms, to form a dry coating on the metal article prior to cold forging. A similar dry coating lubricant is disclosed in U.S. Pat. No. 3,375,193 based on a water soluble colloid, a fatty acid soap having 12 to 22 carbon atoms, an alkali metal tartrate and inorganic pigments.
A glass-forming lubricant is disclosed in U.S. Pat. No. 3,507,791 comprising an aqueous dispersion of a monocarboxylic acid of 10 to 32 carbons, an alkanolamine, a water soluble alcohol and water. The patentee in U.S. Pat. No. 3,983,042 discloses a water-based lubricant for hot forging containing graphite, organic thickener, sodium molybdate and sodium pentaborate.
From the above disclosures, it is apparent that fatty acids and fatty acid soaps have been widely used as anti-wear and lubricant additives in forging compositions. These fatty acids and soaps have generally been preferred in the C.sub.8 to C.sub.20 range. More recently, the Metalprep Department of Pennwalt Corporation has marketed hot forging lubricants containing the alkali metal salt of either azelaic or adipic acid in aqueous solution. Adipic acid salt compositions readily wet the dies at elevated temperatures, e.g., 600.degree.-800.degree. F., are relatively free from smoke and fumes and are stable at elevated temperatures up to about 700.degree. F. At hot forging die temperatures of up to 900.degree. F. (and metal workpiece temperatures of 1200.degree. F. and above) the lubricants normally decompose during the forging process. The importance of providing lubricants with higher temperature stability is to delay decomposition so as to achieve the necessary lubrication before decomposition occurs.
I have now discovered lubricant compositions and a process of hot forging ferrous and non-ferrous metals which provide improved performance with respect to wettability temperature (up to about 900.degree. F.), stability temperature (up to about 800.degree. F.) and lubrication as compared with adipic acid salt compositions.